since the early 1970's when the vulnerability of the United States on foreign energy sources became apparent, greater emphasis has been placed on exploiting our vast coal deposits. Many coal deposits in the West were developed because these deposits could be mined by highly efficient surface mining techniques and because the low sulfur content of these deposits were well suited to meet increasingly stringent environmental demands. Western coal, however, can contain 30% or more moisture which moisture significantly lowers Btu value of these coals and drastically limits their geographic markets.
Over the years many processes have been developed to dry coals, including thermal drying in fluid bed reactors. Fluid bed drying offers many advantages including highly efficient energy and mass transfer and high throughput rates. One problem encountered in the use of fluid bed reactors for coal drying has been the lack of control the fluidization process caused by the non-uniformity of the feed material. Conventional gas distributors consisting of concentric rings or grates of T-bars or other structural shapes, as disclosed in Chemical Engineers' Handbook, Fifth Edition, Editors Perry and Chilton, McGraw-Hill Book Company, New York, 1973, pp. 20-66 to 20-67, permit uneven gas flow and can cause channelling. Gas distributors comprising rigidly mounted perforated metal plates provide more uniform gas distribution but experience clogging by the fluidized solids.